The fast mechanical response of the outer hair cell is believed to be the cellular basis for the positive feedback mechanism required for the fine tuning process of the mammalian hearing organ. We have previously shown that the fast mechanical response of the cell is membrane potential dependent; that a simplified membrane model can describe the elastic property of the cell, and that the membrane capacitance of the cell is dependent on membrane tension as well as membrane potential. This last observation indicates that this motility is based on a membrane motor which directly converts electrical energy into mechanical energy. These observations led to a theoretical model of the cell motility, which predicts the force generation of 0.1 nN/mV for a single cell, which is in agreement with a value of 0.2 nN/mV estimated from in vivo study. The membrane motor requires at least two conformational states. Additional analysis of the motility showed that it is not based on elasticity changes, but on membrane area changes of the membrane motor. A three- state model was proposed to explain currently available observations on capacitance changes. Study of the mechanical property of the cell revealed that it has a viscoelastic relaxation process whose characteristic time constant is about 30 sec. These observations are important for identifying the role of the outer hair cell in the auditory function.